There are methods currently in use that provide varying degrees of success in resolving the collision of the tags, and especially in tags with read/write capability, where communications to the tags is facilitated by an on-board interrogator in the RFID tag. In such a tag, the interrogator can send signals to allow tags to respond with a random number that is manipulated by the interrogator and transmitted to all tags in the field. Only the tag that matches the computed number generated by the interrogator will transmit its data. This process continues until all tags have transmitted their data. This method significantly increases the complexity of the transponder by requiring the transponder to include an interrogator and being only partially effective for regulation of collisions.
Another method of collision resolution is to cause tags to transmit at different frequencies, thereby avoiding a collision of signals. This method significantly increases the complexity of the interrogator while being only partially effective for resolution of the collision.
Yet another method involves the use of spread spectrum techniques. The technique can be either direct sequence spread spectrum (DSSS) or frequency hopping. Either method requires correlation of the signal in the interrogator and requires a very complex interrogator system.
Another method of collision resolution uses part of the unique identification code of the transponder to provide a specific time whereby the transponder transmits its data, and all other times, the transponder remains inactive. This feature is limited by the vast number of unique transmission slots available, and the time required to read all the possible tags in the field. Moreover, a single transponder in the field could take an inordinate amount of time to be read. If the unique identification code is truncated to allow for faster performance, then the probability of an un-resolvable collision occurs due to the duplication of codes.
One other method for resolution is the utilization of a random oscillator and a binary counter on each transponder which enables transmission from the transponder when the counter is in a specific state. All other times the transponder is inactive. This method has the disadvantage of requiring alignment of all the transponders asynchronously before coherent data can be received by the interrogator.
All the above methods require either the use of expensive and complicated interrogator systems, a read-write tag, or they are excessively slow to resolve a useful number of tags.